A device with the use of an organic semiconductor is prepared with a milder film-forming condition than a conventional inorganic semiconductor device, so that the semiconductor thin film can be formed on various substrates and can be formed at ordinary temperature. For this reason, it is expected to reduce the preparation cost and to make the device flexible by forming the thin film on a polymer film or the like.
As for an organic semiconductor material, an aromatic compound has been studied which includes predominantly: a polyacene compound such as anthracene, tetracene and pentacene as well as a conjugate macromolecular compound such as polyphenylenevinylene, polypyrrole and polythiophene, and an oligomer thereof. It has been reported that the polyacene compound particularly has high crystallinity because of having a high intermolecular cohesive force, and thereby exhibits high carrier mobility and excellent characteristics as a semiconductor device.
The form of a polyacene compound to be used in a device has been a vapor deposition film or a single crystal. The device has been investigated to be applied to a transistor, a solar cell, a laser and the like (see Non-Patent Documents 1 to 3).
As for a method to form a thin film of a polyacene compound with a method other than a vacuum deposition method, a method has been reported which applies a solution of a precursor of pentacene, which is one of polyacene compounds, onto a substrate and heats the wet film to form a pentacene thin film (see Non-Patent Document 4). A condensed polycyclic aromatic compound like the polyacene compound has low solubility to a general solvent, is hardly formed into a thin film from its solution by a wet process. Accordingly, the method forms the thin film with the use of the solution of the precursor having high solubility and converts the precursor to the polyacene compound by heat.
On one hand, a polyacene compound having a substituent is described in a report (Non-Patent Document 5) by Takahashi et al., a report (Non-Patent Document 6) by Graham et al., and a report (Non-Patent Document 7) by Mirror et al. Furthermore, Non-Patent Document 8 describes an example of synthesis for 2,3,9,10-tetramethylpentacene, and Non-Patent Document 9 describes an example of synthesis for 2,3,9,10-tetrachloropentacene.
However, examples are not reported in which a thin film of a polyacene compound such as pentacene is formed from a solution of the polyacene compound, and in which a semiconductor device is composed with the use of the thin film. An example of forming the thin film from the solution of an oligothiophene derivative is reported in Patent Document 1.
On the other hand, the performance of an organic semiconductor device provided with a condensed polycyclic aromatic compound thin film has relevance to the purity of the condensed polycyclic aromatic compound, and it is important to increase the purity of the condensed polycyclic aromatic compound (decrease contents of impurities), in order to enhance the performance such as mobility. It has not been easy to obtain an organic semiconductor material of high purity such as a condensed polycyclic aromatic compound, but Patent Document 2 proposes a method of purifying the material with the use of a supercritical fluid.
However, a method of forming the thin film of a polyacene compound with the use of the precursor as described above had a problem that high-temperature treatment is necessary for converting the above described precursor to the polyacene compound (for instance, about 150 to 200° C. for pentacene). In addition, the method concurrently had a problem that an unreacted part remains as a defect because it is difficult to completely convert the precursor into the polyacene compound, and that denaturation occurs due to a high temperature and forms the defect.
In addition, a purifying method according to Patent Document 2 can increase the purity of an organic semiconductor material such as a condensed polycyclic aromatic compound, but a purifying method for further increasing the purity has been demanded in recent years because an organic semiconductor device having higher performance has been required.
For this reason, an object of the present invention is to solve the above described problems of the conventional technology, and to provide an organic semiconductor thin film exhibiting high mobility and a preparing method therefor. Another object of the present invention is concomitantly to provide an organic semiconductor device having excellent electronic characteristics. Still another object of the present invention is concurrently to provide a high-purity condensed polycyclic aromatic compound; a method for purifying a condensed polycyclic aromatic compound; and a method for determining the amount of impurities in the condensed polycyclic aromatic compound.
Patent Document 1: JP2000-307172A
Patent Document 2: JP2003-347624A
Non-Patent Document 1: “Advanced Materials”, Vol. 14, p. 99, 2002
Non-Patent Document 2: Dimitrakopoulos et al., “Journal of Applied Physics”, Vol. 80, p. 2501, 1996
Non-Patent Document 3: Croke et al., “IEEE Transaction on Electron Devices”, Vol. 46, p. 1258, 1999
Non-Patent Document 4: Brown et al., “Journal of Applied Physics”, Vol. 79, p. 2136, 1996
Non-Patent Document 5: Takahashi et al., “Journal of American Chemical Society”, Vol. 122, p. 12876, 2000
Non-Patent Document 6: Graham et al., “Journal of Organic Chemistry”, Vol. 60, p. 5770, 1995
Non-Patent Document 7: Mirror et al., “Organic Letters”, Vol. 2, p. 3979, 2000
Non-Patent Document 8: “Advanced Materials”, Vol. 15, p. 1090, 2003
Non-Patent Document 9: “Journal of American Chemical Society”, Vol. 125, p. 10190, 2003